Background: Nanaocrystalline zinc oxide (ZnO NP) with a tunable morphology has been synthesised by means of a novel sol–gel method, and exhibits enhanced photophysical properties, high cytotoxicity, antibacterial, abated toxicity, and significant excitation energy. The presence of unreacted metabolites like carbohydrates, flavonoids, and gallic acid in the filtrate of the aqueous leaf extract of Psidium guajava (P) and Azadirachta indica (A) leaf aids in the green synthesis of ZnO NPs.

Methods: In the initial step, a 40 mL aqueous leaf extract from two plants is combined with 460 mL of 4.36 M zinc acetate and heated at 80 °C while stirring. A reddish-brown precipitate forms and is filtered, allowing the remaining filtrate to be used for the chemo-green synthesis of ZnO NPs. Then, 20 mL of 1 M NaOH is separately added dropwise in 480 mL of the filtrate of P plants and A plants, with constant stirring for 20 minutes at room temperature and then stirring at 80 °C for 6 hours. The resulting ZnO NPs are filtered, washed with ethanol, subjected to calcination at 4500C for 2 hours and characterised by XRD, DRS, and Zeta potential for crystalline, optical, and surface charge. For photocatalytic testing, a 100 mL solution of reactive blue-171 at 10 ppm is stirred, followed by exposure to sunlight. Additionally, the antimicrobial efficacy against Gram-negative Escherichia coli is evaluated.

Results and Conclusion:

Synthesised ZnO NPs from P and A have direct band gap energies of 4.72 and 4.611 eV and negative zeta potentials of 22.4 and 23.4 mV, and lead to 89.2% and 96.4% dye degradation in 140 minutes, respectively. The antibacterial activity of A is superior to that of P. This study suggests that the repurposing of waste filtrate by the green method for the high-yield synthesis of ZnO NPs with scant use of an alkali fosters sustainability and alleviates economic strains.